PhysicsLAB Worksheet
Doppler - A Challenge Problem

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A police car is parked on the side of the road using a "speed gun". His assignment is to clock the speed of oncoming cars based on a chart comparing frequency differentials (between his gun's original frequency and the frequency received from the wavefront reflected from the oncoming car) and car speeds.
Refer to the following information for the next seven questions.

A stationary source directs an 800-hz wave toward an approaching object that is traveling at a speed of 25.0 m/s. Our task is to determine the frequency shift between the reflected wave and the original signal.
True or False. There are two Doppler shifts here.
Prior to striking the approaching object, which Doppler equation would you use: that for a moving source or the one for a moving listener?
True or False. If the air temperature is 20°C, the apparent frequency striking the approaching object would be 859 hz.
The reflected sound wave would appear to be originating from a moving source that has what value as its original wavelength? 

How much does the wavelength change as a result of the motion of the approaching object?
What is the frequency shift as heard by the original stationary source when it receives the reflected wave? 

What would have been the frequency differential had the car been traveling at 50.0 m/sec towards the stationary police officer? 

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Catharine H. Colwell
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